Moon Phases

The Full Harvest Moon occurred last night at 6:31pm, but you can still observe it for the next couple of days.  The moon takes 29.53 days to complete one full orbit around the earth.  During that time it passes through eight phases, each lasting a little less than 4 days.  A phase is determined by where each of three bodies are positioned: the sun, the moon, and the earth.  The angle of the moon from each of the other two determines what phase of the moon we will see.  What we see of the moon is due to sunlight reflected from the surface of the moon back to the earth.

The moon does not rotate as it proceeds along its orbit around the earth, so we are always looking at the same side of the moon.  There is an excellent presentation of how the phases of the moon are seen from earth provided at: https://www.timeanddate.com/astronomy/moon/location.html.  The phases include:

1. New Moon:  the moon is between the earth and the sun, so no light is reflected back to the earth and the moon is not visible. 
2. The Waxing Crescent is visible as the moon moves eastward and an observer starts to see a small slice of the moon’s surface.  As the visible crescent grows larger each night, it is called waxing which means to increase gradually.
3. First Quarter is when the moon is one-quarter of the way around its orbit.
4. The Waxing Gibbous Moon is more than half full but not totally full.
5. The Full Moon shows us the completely lit side that faces the earth
6. The Waning Gibbous Moon is still more than half lit, but waning refers to a gradual decrease in the visible side.
7. The Third Quarter Moon is when the moon is three-quarters of the way around, now located on the other side of the night sky.
8. The Waning Crescent phase shows the remaining small slice of the moon before it once again moves to a New Moon and the lighted surface is no longer visible to an observer.

As you can see from the presentation on the web site above, the moon’s orbit is elliptical, but sometimes it is closer to earth, sometimes farther.  The path that it follows is called the Metonic Cycle, and it is repeated every 19 years.  It was discovered in 432 B.C. by the Greek astronomer, Meton of Athens.  Due to slight variations in the moon’s travel as well as the slowing rotation of the earth, the cycle may be off by 1 day in some 19-year periods and then return to the same day in other periods.

When there are two full moons in a month, we call the second one a Blue Moon.  Blue Moons occur once every two to three years.  This month, the Blue Moon will appear on Halloween night, an occurrence that will not happen again for another 19 years.  The exact time of the Blue Moon will be October 31, 2020 at 9:49 am.

Folklore from Native Americans, settlers, and indigenous people of western Europe has introduced many other names for each of the monthly full moons throughout the calendar year:

• January – Wolf Moon: Native Americans believe that wolves hunt around villages on cold winter nights and howl at the full moon.
• February – Snow Moon: This marks the month of the heaviest snowfalls.
• March – Worm Moon: As the ground thaws, earthworm start to appear.  Settlers colonizing early U.S. areas also call this the Lenten Moon, marking the last full moon of winter.
• April – Pink Moon: Spring rains bring wildflowers into bloom including an herb called ‘moss pink’ or ‘wild ground phlox,’ Phlox subulata.
• May – Flower Moon: It marks the spring time bloom.
• June – Strawberry Moon: Strawberries are only ripe for a short period at this time of year.
• July – Buck Moon: It marks when antlers covered in velvet are first seen on mature bucks.
• August – Red Moon: Due to the haze caused by high humidity, the moon takes on a red tinge.
• September – Harvest Moon: The full moon that occurs closest to the autumnal equinox when the sun’s trajectory crosses the equator and nights become longer than days.
• October – Hunter’s Moon: Now is the time for hunting fattened deer for winter stores.  Hunter’s Moon is an important feast day for Native Americans and most of western Europe.
• November – Beaver Moon: It marks the time to set traps for beaver, before winter freezes all of the waterways.
• December – Long Night’s Moon: At this time of year, the moon has the longest and highest trajectory above the horizon.

All solar bodies including stars, planets and moons have gravity.  Gravity extends outward from every solar body for some distance depending on the mass of each solar body and how far two bodies are apart.  The moon’s gravity pulls at everything on the earth, effectively stretching the earth toward the moon and squashing the earth at right angles to the moon.

As water is also pulled and squashed, a bulge in the world’s oceans develops on the side of the earth facing the moon as well as on the opposite side.  The earth rotates under each bulge, and high tide occurs as the bulge passes over a shoreline.  Thus, there are two high tides each day.

As the earth rotates under the oceans, friction with the water slows down the earth’s rotation a fraction of a second each century.  This does not really affect us, but in 100 million years, a day will last 30 minutes longer.  At the same time, the moon’s orbital speed is increasing a fractional amount.  In that same 100 million years, the moon will spiral ever farther from the earth, and it will be an average of 3000 kilometers (1864 miles) farther away from the earth.  As we look into the past, these same effects have been present since the moon was created some 4.5 billion years ago. 

An example of the effect this friction has had on earth and the moon comes from a study of coral reefs.  A coral reef is made up of millions of tiny animals called polyps.  Each day, polyps deposit a layer of calcium to a coral reef.  Scientists have been able to count the layers of calcium to determine the age of a reef, including fossilized coral from the Devonian period.  When counting the layers of fossilized coral, the number of layers present is much higher than the sum total of days in that period.  We can account for this difference by calculating how much faster the earth was rotating in the past.  The results show us that a day in the Devonian period lasted about 22 hours, and there were 400 days in a year accounting for the extra layers of calcium in the fossilized coral.

Luna, from the Latin lucere, meaning to shine was the Roman goddess of the moon, animals and hunting.  Her symbol is a crescent  ☾.  The word luna and the word moon were used in common language for many hundreds of years to describe our neighboring satellite.  In 1919, the first meeting of the newly formed International Astronomical Union was held, and the name was standardized to “moon”.  This month we will experience all phases of the moon, including a Harvest Moon and a Blue Moon.  You can track moonrise, moonset and phases by date and time, plus lots of other lunar information, from the Time and Date website.  I hope you find Halloween to be an enjoyable and interesting time this year.